1. Field of the Invention
The invention relates to an actuator for a valve, in particular a turbine valve, with a valve spindle for setting an opening position of the valve and with a drive part assigned to the valve spindle.
2. Description of the Related Art
A turbine, in particular a steam turbine, normally has a considerable number of valves which may be used, for example, as fresh-steam, interception or diversion valves and as quick-action stop valves. The opening position of each of these valves serves in this case for setting a respective material, gas or steam stream and can be set, for example, via a valve spindle assigned to the respective valve. Such a valve spindle is an integral part of an actuator assigned to the valve. The actuator may in this case have to satisfy very high requirements, particularly in terms of actuating force and actuating speed. For a high reliability of the valve, it may be necessary, for example, for the actuator to have an actuating force of about 200 kN and an actuating time of about 100 ms, particularly when there is a quick-action stop requirement.
The valves are therefore conventionally designed as oil-hydraulically operated actuating valves, the actuators assigned to them including in each case an oil-hydraulic device. For operating the actuators and therefore the valves, for example, a central hydraulic supply system may be provided. For a central hydraulic supply system of this type, however, it is necessary to have a complex and therefore cost-extensive pipeline system which should also be designed redundantly for high operating reliability. To simplify a complex supply system of this type, European patent application EP 0 040 732 A1 and the paper by W. Kindermann, E. G. Egener and H. Tremühlen, “Compact Valve Actuator Control System for Large Steam Turbines”, presented at the American Power Conference, Chicago, 1984, disclose actuators for steam turbine valves which in each case have a decentralized hydraulic system. The hydraulic system of an actuator of this type is integrated into a compact drive block arranged on a valve housing, so that only a cable system is still necessary in order to supply energy to the actuator. By oil being used as hydraulic fluid, an ignition of the oil and therefore a fire in the steam turbine could occur under extremely unfavorable circumstances. Admittedly, with a view to fire protection, low-combustible fluids may also be used as hydraulic fluid. However, low-combustible hydraulic fluids of this type are costly and, because of their lower stability, as compared with hydraulic fluids based on mineral oils, require comparatively complicated care measures.
DE-A-1 937 198 describes a regulating valve for regulating the pressure and/or the quantity of a flowing medium in power stations, chemical plants or the like. The regulating valve has a push rod, to which a valve seat is fastened. The push rod is coupled to an electric servo-motor via a beam rotatable about a central center of rotation. The quantity of medium flowing through the valve can thereby be set. Furthermore, a return spring engages on the push rod.
EP 0 230 849 A1 specifies a regulating valve and a shut-off valve. The valve has a shut-off body which is displaceable along an axis and which has a piston extending along the axis. The piston is designed as a spur-toothed rack which is in contact with a longitudinal toothing of a further bar. As a result of a rotation of this further bar about its bar axis, a displacement of the shut-off body in the axial direction can be achieved. A rotation of this bar takes place by a further toothing at the end of this bar via an electric motor.
DE 44 46 605 A1 discloses a valve for a steam turbine which has a valve spindle with a valve cone arranged on it. The valve spindle is driven via an electric motor which is connected to the valve spindle via an electromagnetically actuated coupling. For automatic self-closing of the valve, the latter includes a cup-spring system. The electromagnetic coupling is connected to a threaded bush which cooperates with a valve spindle guided fixedly in terms of rotation and which thus moves the latter axially. The threaded bush is designed as a ball-screw bush, so that it acts upon the valve spindle with low play and low friction. Since the valve spindle is guided fixedly in terms of rotation, that is to say it can only be moved up and down axially when the threaded bush rotates, it is not necessary to have a drive which has the effect of translational motion. Instead, an electric motor rotating in two directions is sufficient for this purpose. However, the electric motor must have a torque safeguard, in order thereby to ensure that, for example during the closing of the valve, the valve cone adjoining the valve spindle at one end is not damaged or does not damage the sealing seat (valve seat) assigned to the valve cone when the latter comes to bear on the sealing seat.
WO 98/13633 specifies an actuator for a valve for a turbine, the opening position of which valve can be set by a push rod, a fault-free operation of the valve being ensured, along with a particularly low fire risk. For this purpose, an electric motor is provided for driving the push rod. The push rod is connected to the electric motor via a gearwheel/rack system and via an electromagnetic toothed coupling. For reasons of operating reliability for the actuator and therefore also for the valve capable of being driven by the latter, in WO 98/13633 a retaining-spring system is also arranged on the push rod. The system includes a retaining spring which is arranged in a housing and which acts on a thrust plate fastened to the push rod. The actuator is in this case designed in such a way that the spring force of the retaining spring causes a closing of the valve. In this case, for opening the valve, the electric motor acts counter to the spring force of the retaining spring.
An electromechanical actuator for a valve, in particular for a steam turbine valve, is specified in WO 99/49250. The actuator has a push rod and an electric motor for driving the push rod. The push rod and the electric motor are connected via a transmission device, by which a changing torque can be generated, depending on the axial displacement of the push rod. The push rod is connected to a return-spring system. In this case, even in the closing position of the valve, the push rod is loaded by the return force of the return-spring system which is designed as a valve cup spring accumulator assembly. During an opening movement of the actuator, the return-spring system is further tensioned and reaches its maximum return force when the valve is in the open position. Consequently, in a similar way to what is described in WO 98/13633, during the setting of an opening position of the valve, the drive has to perform work counter to the considerable return force of the return-spring system.